This invention relates to pilferage detection systems, and more particularly to pilferage detection systems in which a magnetic marker placed in or on an article subject to pilferage is detected by detection circuitry if the article is removed from a protected area unless the marker is first removed from the associated article or deactivated.
The problem of pilferage of merchandise from retail stores, books from libraries, and the like is well known. Many different types of systems have been devised in an attempt to deal with this problem. These systems have met with varying degrees of success. Among the most promising pilferage detection systems are those in which a magnetic "marker" of any of several types is placed in or on articles subject to pilferage. Unless the marker is removed or modified in some way, presumably when an article is authorized for removal from the protected area (e.g., sold, in the case of merchandise in a store, or checked out, in the case of books in a library), the marker is detected as the article is carried to or through the exit from the protected area.
Among the earliest systems of this type are those shown in French patent 763,681 issued to P. A. Picard in 1934. In the Picard systems, a transmitting antenna coil is driven by an alternating current (AC) signal having a predetermined fundamental frequency. A receiving antenna is disposed adjacent the transmitting antenna and both antennas are located near the exit from a protected area so that a person leaving the protected area must pass through the electromagnetic field set up by the transmitting antenna. The transmitting and receiving antennas are arranged so that there is normally no net signal induced in the receiving antenna (i.e., the transmitting antenna is balanced relative to the receiving antenna). When a person enters the electromagnetic field of the transmitting antenna carrying a piece of magnetic material, the balance of the transmitting antenna is disturbed and a net signal is induced in the receiving antenna. The nature of the induced signal depends on the characteristics of the magnetic material. According to Picard, if the magnetic material is of moderate permeability (e.g., iron, steel, or nickel) and is capable of being saturated by the field of the transmitting antenna, the induced signal exhibits the fundamental frequency and several lower order odd harmonics of the fundamental frequency (e.g., the third and fifth harmonics of the fundamental frequency). If, on the other hand, the magnetic material is of high permeability (e.g., Permalloy, mu-metal, or Permafy), the induced signal also includes higher order odd harmonics of the fundamental frequency (e.g., the ninth, eleventh, etc., harmonics). By appropriately filtering the signal induced in the receiving antenna, the presence of particular magnetic materials can be detected by the presence of particular odd harmonics of the fundamental frequency in the induced signal. Since most people do not ordinarily carry materials having the magnetic characteristics of Permalloy, Picard proposes the use of a piece of Permalloy in or on articles as a marker to detect pilferage of those articles. Detection of one or more of the higher order odd harmonics characteristic of Permalloy in the signal induced in the receiving antenna can then be used to indicate that an article with a marker is being removed from the protected area.
U.S. Pat. No. 3,665,449 issued to J. T. Elder et al on May 23, 1972 shows pilferage detection systems in which magnetic markers composed of one, two, or more elements are employed to produce signals in a high frequency band (e.g., above 1000Hz) when subjected to a low frequency alternating magnetic field (e.g., 60Hz). The Elder et al systems do not detect particular harmonics of the fundamental frequency, but rather detect all frequencies in a given band. Where a marker includes two or more elements, Elder et al suggest that these elements can be of different permeabilities to produce output signals even more complex and distinctive than those produced by a marker of substantially uniform permeability. Elder et al also suggest that one element of a marker having two or more elements can be a "control" element which is remanently magnetizable. When the control element is demagnetized, the marker is sensitized or activated (i.e., produces the characteristic output signals associated with the reversal of magnetic polarity by the other marker element or elements). When the control element is magnetized, the marker is desensitized or deactivated (i.e., the other marker element or elements are prevented from reversing polarity and therefore produce no output signal, or reverse polarity in such a different fashion that the output signal is not recognized as that of an active marker).
U.S. Pat. No. 3,631,442 issued to R. E. Fearon on Dec. 21, 1971 and U.S. Pat. No. 3,747,086 issued to G. Peterson on July 17, 1973 (a "division" of the application on which the Fearon patent issued) show pilferage detection systems similar to those discussed above and employing magnetic markers having three elements, two of which are remanently magnetizable control elements (see, for example, FIG. 11 of the Fearon patent). As described by Fearon and Peterson, such markers have a number of possible states depending on the magnetization of the control elements. In general, magnetization of the control elements causes the marker to produce even as well as odd harmonics of an applied fundamental frequency. Fearon and Peterson therefore suggest determining the state of the marker by detecting a ratio of selected even and odd harmonics of the fundamental frequency. If both control elements are left strongly magnetized in the same direction, the marker is silent (i.e., the polarity of the third element does not change in response to the applied field) and the marker cannot be detected (i.e., the marker is deactivated). Peterson also describes a system employing a magnetic marker having two elements, one of which is remanently magnetizable (see column 12, lines 40-66 of the Peterson patent). In this embodiment, as described by Peterson, the marker produces detectable odd harmonics of the fundamental frequency if the control element is unmagnetized and is silent or deactivated if the control element is magnetized.
There are various defects associated with all of the foregoing systems. In the Picard system the marker is not controllable (i.e., there is no means of deactivating a marker). The marker must therefore be either removed or destroyed when the associated article is authorized for removal from the protected area or some other means must be provided for permitting authorized removal of articles from the protected area. If the marker is to be removed or destroyed, it must be placed on the protected article where it can be easily located. In general, this will make it possible for anyone to locate and tamper with the marker. The Picard system may also give false alarms in response to large pieces of magnetic materials other than Permalloy tags. The systems shown by Elder et al employ extremely complicated receiving apparatus including both frequency-domain and time-domain filtering. In addition, the Elder et al markers employing remanently magnetizable control elements can be deactivated or silenced completely by magnetizing the control elements. Magnetization of a control element is a relatively simple operation, requiring only the manipulation of a sufficiently strong magnet. Accordingly, it may be relatively easy to tamper with these markers using a simple magnet. Accidental demagnetization of the control elements of these markers may also occur in the presence of large magnetic or electromagnetic fields such as those frequently occurring near electric motors and other electrical or electronic appliances. This may result in reactivation of deactivated markers, thereby giving rise to false alarms. The Fearon and Peterson systems employing markers with magnetizable control elements are equally subject to unauthorized deactivation through the use of magnets and accidental reactivation as a result of demagnetization of the control elements. Moreover, in any system such as the Elder et al, Fearon, or Peterson systems in which a marker is deactivated by magnetizing one or more control elements, the control elements must generally be magnetized parallel to the longitudinal dimension of the other marker elements. This means that the marker must be physically located or its orientation otherwise determined before the control element or elements can be properly magnetized to deactivate the marker. This greatly complicates the deactivation procedure or the apparatus required to perform a deactivation procedure. It is an important advantage of the systems of this invention that marker deactivation is accomplished by demagnetizing the control element of a marker and that this can be accomplished without physically locating the marker and substantially without regard for the orientation of the marker relative to the deactivation apparatus.
It is therefore an object of this invention to improve and simplify pilferage detection systems employing magnetic markers.
It is a more particular object of this invention to provide pilferage detection systems employing magnetic markers which are less subject to being tampered with by magnets.
It is another more particular object of this invention to provide pilferage detection systems employing magnetic markers with reduced sensitivity to accidental interference by other electrical apparatus in the environment of the protected articles or the pilferage detection apparatus, and with reduced sensitivity to interference from other passive but magnetically non-linear objects that are likely to pass through the detection field.
It is still another more particular object of this invention to provide pilferage detection systems employing magnetic markers which can be deactivated without physically locating the marker and substantially without regard for the orientation of the marker relative to the deactivation apparatus.